Field of the Invention
This invention relates to a receiving apparatus for automatically setting reception characteristics in data communication to absorb the effects of, e.g., line jitter. The invention also relates to a transmitting-receiving apparatus for use in data communication.
When digital signal data is transmitted via an ordinary pay station line, which is an analog line, it is required that the digital data be modulated and converted into the desired analog signal before being sent, and it is required that the modulated signal be demodulated on the receiving side. Modems for this purpose are essential in both the transmitting apparatus and receiving apparatus.
In presently existing GIII-type Facsimile machines in which the data transmission rate has been raised, data can be transmitted at a high speed, namely at an information transmission speed of 9600 bps (bits/sec). The signal modulated by the modem on the transmitting side and then transmitted over a line is received by the receiving apparatus but on the way is distorted due to line distortion, jitter, transmitting-receiving timing error, carrier error and the like. This signal is received by the modem on the receiving side. Such modem incorporates an equalizer which corrects for this distortion. As a result, the corrected signal outputted from the modem on the receiving side is the original transmitted signal.
The operation of this equalizer will now be described with reference to FIG. 12 and FIGS. 13(a) through (c).
In FIG. 12, numeral 10 denotes the modem on the transmitting side, 20 the modem on the receiving side, 21 the equalizer, which is incorporated in the receiving-side modem 20, and 30 a line connected the two modems.
The line 30 has a frequency characteristic, and the transmission characteristic thereof is as shown in FIG. 13(b), by way of example. Even if a signal aK having the frequency characteristic shown in FIG. 13(a) is transmitted by the modem 10 on the transmitting side, a signal Rk received by the receiving-side modem 20 is influenced by the transmission characteristic and takes on, for example, the form shown in FIG. 13(b). When the receiving-side modem 20 is provided with the equalizer 21 having a frequency characteristic (FIG. 13c) which is the inverse of the frequency characteristic of line 30, the composite characteristic of the line and equalizer [a characteristic obtained by superimposing these two characteristics (simple multiplication in the frequency domain)] becomes the flat characteristic shown in FIG. 13(a). As a result, distortion-free signal transmission becomes possible.
Thus, the function of the equalizer 21 is to produce a characteristic which is the inverse of the line characteristic.
Ordinarily, in order to produce the equalizing characteristic of the equalizer, normalized data referred to as training data is transmitted in advance of data transmission, and the equalizing characteristic is set automatically while the training data is being accepted.
However, in the prior art described above, the automatic setting is performed by computing the equalizing characteristic each time one bit of the training data is received, by way of example. Consequently, a comparatively long training time is required to complete the automatic setting.
The length of the training time is not a particular problem when data is transmitted to only one location. However, in a case where the same data is sent simultaneously, to a plurality of locations, e.g., to 20 or 30 locations, the length of training time has a major influence upon overall transmission time and tends to impede rapid transmission.
A polling system illustrated in FIG. 14 may be considered as one example of a signal transmission application which uses transceivers (modems) connected to a line having different transmission frequency characteristics.
This system, which may be thought of as an on-line service system such as a cash dispenser at a bank or the like, includes a master station A and a plurality of slave stations B, C, D, E . . . interconnected by a transmission line 120. The master station A is incapable of simultaneously receiving signals from the plurality of slave stations B through E. The transmission line 120 is used on a time-shared basis so that a signal can be received from one of the slave stations in response to a request from this slave station. A modem is disposed at the transmission line terminal of each station.
Since the slave stations are located at different distances from the master station A, the transmission frequency characteristics between the master station and each slave station differ. However, it is required that a request from each slave station be responded to quickly.
In a case where an extended period of time is needed to respond to a request from a slave station, a request from another slave station cannot be accepted, request waiting time increases and efficiency declines. Moreover, line utilization time increases and, as a result, so does the number of lines used.
In particular, since the master station A in the conventional system can only perform a fixed equalizing operation regardless of which slave station issues a signal, as mentioned above, equalizing speed is required and efficiency suffers.
Furthermore, phase jitter generated in the transmission line is a component that cannot be removed by the equalizer itself. Phase jitter is a phenomenon in which the phase of the transmitted data revolves (i.e., is phase modulated) on the line due to a commercial frequency (50 Hz, 60 Hz) component. As far as the equalizer is concerned, this is a very rapid fluctuating component which cannot be absorbed by the conventional equalizer. That is, since the equalizer outputs the results of computations using data in a certain range in terms of time, the output itself contains all of the jitter component in the same range of time.
In order to remove such a fluctuating component, the prior art adopts a jitter suppression method based on the arrangement shown in FIG. 15. This conventional jitter suppression method will now be described with reference to FIG. 15 showing the equalizing section of the prior art.
As shown in FIG. 15, the conventional equalizing section includes an equalizer 114 for eliminating received data distortion, a decision unit 115 for judging data transmitted from the output of the equalizer 114, a phase detector 150 for extracting a jitter component from the equalized output signal, and a phase controller 151. This arrangement corrects for jitter by means of feedback control. The reason for correcting jitter at this position is that the jitter component can be easily detected since line distortion is equalized by the equalizer 114.
Since the influence of jitter appears as a revolving component of the data, rotation in the direction opposite that of the jitter phase detected by the phase detector 150 is applied by a phase rotating unit 152, whereby the jitter component is eliminated. This phase rotation is carried out by multiplying the output of the equalizer 114 by a wave having a predetermined phase.
Accordingly, with the example of conventional control shown in FIG. 15, phase control is applied in dependence upon the amount of jitter detected from the equalized output. As a consequence, the amount of phase controlled is merely an average value seen in the time span of the equalizer. This means that the jitter component cannot be completely removed. As a result, an error component cannot be made smaller than a certain value, thereby bringing about a deterioration in transmission quality.